Exclude Python version 3.6 in CI

[LauWien]

With issue actions/runner-images#4060 macOS-latest workflows will use macOS-11, saying "EOL Python versions were deprecated" and the end-of-life of Python version 3.6 being the 23rd of December 2021, using Python version 3.7 instead of 3.6 might make sense. Possible also in message_ix.

[khaeru]

The README for actions/setup-python seems to indicate that earlier versions will continue to be available; they'll just not be pre-installed on the runners. This means the setup-python action will need to download and install 3.6, which might be a little slower. At least, that's my interpretation; we should see if that does work on the macos-11 runners.

In general, the policy is:

• The minimum Python version for ixmp can't be earlier than the minimum Python version of its mandatory dependencies (or their mandatory dependencies).
• When those packages remove support or enforce a minimum version, we also remove support.
• We try our best (within reason) to test the minimum version (per the above 2 points) on CI.